Bitumen is used as a binder in road asphalt mixtures and has been developed continually to meet ever-increasing performance demands from road building constructors. In general, bitumen performs well in road asphalt but increasingly heavy traffic loads have led to the premature wear of many roads through rutting and cracking of the surface. Cracking is a serious defect in a road asphalt because it allows water to reach lower layers of the road surface where it causes rapid deterioration and accelerates the need for repairs. Increasing the bitumen content of asphalt or using a softer grade of bitumen improves the crack resistance of asphalt at low temperatures but increases the risk of excessive rutting at higher temperatures because the mixture is effectively softer. Conversely, resistance to rutting can be improved by reducing the amount of bitumen in the asphalt mixture or by using a harder grade of bitumen at the expense of crack resistance because the mixture becomes less flexible.
Much effort is therefore nowadays directed to the development of a hard bitumen composition meeting today's crack resistance requirement, i.e. a bitumen composition having both a good low temperature performance and a good high temperature rutting resistance.
Another requirement which bitumen compositions frequently have to meet nowadays is the resistance to solvency of petroleum products such as fuels (e.g. gasoline and kerosene) and oils such as lubricating oils. This resistance to solvency is of importance in areas where asphalt mixtures are regularly exposed to spillage of fuels and oils. Such areas include for instance airfields, waste disposal areas, gas stations, and tank storage areas. When an asphalt mixture is exposed to fuel or oil spillage, the bitumen will be leached out of the asphalt mixture, eventually resulting in the loss of coarse aggregate from the road surface, so-called fretting of the road surface.
In view of the above it will be clear that it would be advantageous to develop a hard bitumen composition meeting today's crack resistance requirement and which would in addition display an improved resistance to solvency of petroleum products.